Material processing apparatus, such as plasma arc torches and lasers are widely used in the cutting of metallic materials. A plasma arc torch generally includes a torch body, an electrode mounted within the body, a nozzle with a central exit orifice, electrical connections, passages for cooling and arc control fluids, a swirl ring to control the fluid flow patterns, and a power supply. Gases used in the torch can be non-reactive (e.g., argon or nitrogen), or reactive (e.g., oxygen or air). The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum.
Plasma arc cutting torches produce a transferred plasma arc with a current density that is typically in the range of 20,000 to 40,000 amperes/in2. High definition torches are characterized by narrower jets with higher current densities, typically about 60,000 amperes/in2. High definition torches produce a narrow cut kerf and a square cut angle. Such torches have a thinner heat affected zone and are more effective in producing a dross free cut and blowing away molten metal.
Similarly, a laser-based apparatus generally includes a nozzle into which a gas stream and laser beam are introduced. A lens focuses the laser beam which then heats the workpiece. Both the beam and the gas stream exit the nozzle through an orifice and impinge on a target area of the workpiece. The resulting heating of the workpiece, combined with any chemical reaction between the gas and workpiece material, serves to heat, liquefy or vaporize the selected area of the workpiece, depending on the focal point and energy level of the beam. This action allows the operator to cut or otherwise modify the workpiece.
Certain components of material processing apparatus deteriorate over time from use. These “consumable” components include, in the case of a plasma arc torch, the electrode, swirl ring, nozzle, and shield. Ideally, these components are easily replaceable in the field. Nevertheless, the alignment of these components within the torch is critical to ensure reasonable consumable life, as well as accuracy and repeatability of plasma arc location, which is important in automated plasma arc cutting systems.
Some plasma arc torches include a liquid cooled electrode. One such electrode is described in U.S. Pat. No. 5,756,959, assigned to Hypertherm, Inc. The electrode has a hollow elongated body with an open end and a closed end. The electrode is formed of copper and includes a cylindrical insert of high thermionic emissivity material (e.g., hafnium or zirconium) which is press fit into a bore in the bottom end of the electrode. The exposed end face of the insert defines an emission surface. Often the emission surface is initially planar. However, the emission surface may be initially shaped to define a recess in the insert as described in U.S. Pat. No. 5,464,962, assigned to Hypertherm, Inc. In either case, the insert extends into the bore in the bottom end of the electrode to a circulating flow of cooling liquid disposed in the hollow interior of the electrode. The electrode can be “hollowmilled” in that an annular recess is formed in an interior portion of the bottom end surrounding the insert. A coolant inlet tube having a hollow, thin-walled cylindrical body defining a cylindrical passage extending through the body is positioned adjacent the hollow interior surface of the electrode body. The tube extends into the recess in a spaced relationship to provide a high flow velocity of coolant over the interior surface of the electrode.
In many plasma arc torches and under a variety of operating conditions (e.g., high amperage cutting), the tube must remove the heat from the electrode by providing sufficient cooling to obtain acceptable electrode life. It has been empirically determined that if the outlet of the coolant tube is misaligned (longitudinally and/or radially) with the interior surface of the electrode, the tube does not sufficiently cool the insert. Repeated use of a torch having a coolant tube misaligned with the electrode causes the insert material to more rapidly wear away. To achieve desirable coolant flow characteristics, the tube is typically secured in a fixed position relative to the electrode to achieve proper alignment. Electrode wear typically results in reduced quality cuts. For example, the kerf width dimension may increase or the cut angle may move out of square as electrode wear increases. This requires frequent replacement of the electrode to achieve suitable cut quality.
Tolerances associated with conventional methods of mounting the electrode and coolant tube makes it more difficult for systems employing such torches to produce highly uniform, close tolerance parts without requiring frequent replacement of the electrode due to the errors inherent in positioning the electrode relative to the coolant tube.
It is therefore a principal object of this invention to provide electrodes and coolant tubes for a liquid-cooled plasma arc torch that aid in maintaining electrode life and/or reducing electrode wear by minimizing the effects of misalignment.